The following text discusses how atmospheric conditions affect the forecasted intensity of high pressure systems.

. Highs undergo little or no change in intensity when isotherms and contours are symmetrical . Highs intensify when warm air advection occurs on the west side of the high.

. Highs weaken when cold air advection occurs on the west side of the high.

. Blocking highs usually intensify during westward movement and weaken during eastward movement.

. Convergence and height rises occur in the downstream trough when high-speed winds with a strong gradient approach low-speed winds with an anticyclonic weak gradient. This is often the case in ridges where the west side contains the high-speed winds; the ridge intensifies due to this accumulation of mass. This situation has also been termed overshooting. This situation can be detected at the 500-hPa level, but the 300-hPa level is better suited because it is the addition or removal of mass at higher levels that determines the height of the 500-hPa contours.

. Rise and fall centers on the time differential chart indicate the changes in intensity, both sign (increasing, decreasing) and magnitude of change, if any, in decimeters. The magnitude of the height rises or falls can be adjusted if other indications reveal that a slowing down or a speeding up of the processes is occurring, and expected to continue.

The following text discusses how atmospheric conditions affect the forecasted intensity of low-pressure systems.

. Lows and cutoff lows deepen when cold air advection occurs on the west side of the trough.

. Lows fill when warm air advection occurs on the west side of the low.

. Lows fill when a jet maximum rounds the southern periphery of the low.

. Lows fill when the jet maximum is on the east side of the low, if another jet max does not follow.

. Lows deepen when the jet max remains on the west side of the low, provided the jet max to the west of the low is not preceded by another on the southern periphery or eastern periphery of the low, for this indicates no change in intensity.

. The 24-hour rise and fall centers aid in extrapolating both the change and the magnitude of falls in moving lows. Again, these rise and fall indications must be considered along with advection factors, divergence indications, and the indications of the contour-isotherm relationships.

FORECASTING THE FORMATION OF UPPER LEVEL AND ASSOCIATED SURFACE FEATURES

The following text deals with the formation of upper level and associated surface features, and how atmospheric features affect them.

The following are atmospheric condition indicators that are relevant to the formation of highs.

. Cold air masses of polar and Arctic origin generally give no indication of the formation of highs at the 500-hPa level or higher, as these airmasses normally do not extend to this level.

. The shallow anticyclones of polar or Arctic origin give indications of their genesis primarily on the surface and the 850-mb charts. The area of genesis will show progressively colder temperatures at the surface and aloft; however, the drop in the 850-mb temperatures does not occur at the same rate as at the surface. This is an indication that a very strong inversion is in the process of forming. The air in the source region must be relatively stagnant.

. High-level anticyclogenesis is indicated when low-level warm air advection is accompanied by stratospheric cold air advection. This situation has primary application to the formation of blocks, as high-level anticyclogenesis is primarily associated with the formation of blocks and the intensification of the ridges of the subtropical highs.

. Blocks should normally be forecast to form only over the eastern portion of the oceans in the middle and high latitudes. Warm air is normally present to the north and northwest.

There are certain conditions required in the atmosphere, as well as certain atmospheric indicators, for cyclogenesis to occur. The greater the number of these indicators/conditions in agreement, the greater the success in forecasting cyclogenesis. Some of them are listed below:

. An area of divergence exists aloft.

. A jet maximum on the west side of a low indicates deepening and southward movement.

. Cold air advection in the lower troposphere and warming in the lower stratosphere is associated with the formation of or deepening of lows.

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